In a well completion, a string of casing or pipe is set in a well bore and cement is forced into the annulus between the casing and the well bore, primarily to separate oil and gas producing horizons from each other and from water-bearing strata.
If the cement fails to provide a separation of one zone from another, then fluids under pressure from one zone may be able to migrate and contaminate an otherwise productive nearby zone. Migration of water in particular produces undesirable water cutting of a producing zone and possibly can make a well noncommercial.
Cement failures can occur in a variety of manners. For example, there may, for one reason or another, be a complete absence of cement behind the casing segment where the cement should be. This would be a gross cement bonding failure leading to rapid contamination between zones intended to be separated.
Another type of cement failure arises when the cement is present behind the casing, but a small cement-free annulus exists between the cement and casing. This annulus may be so thick as to enable hydraulic communication between zones leading to undesirable contamination.
Such annulus, however, may also be so thin as to effectively preserve the hydraulic security function of the cement. Such acceptable small annulus may arise from the technique employed to introduce the cement in the first place. For example, the cement typically is introduced under very high pressure such as produced by using a heavy mud to chase the cement plug down and into the annulus around the casing. The resulting pressure inside the casing causes a slight expansion of the casing and subsequent contraction when the heavy mud is removed. The magnitude of the contraction depends upon the pressure and casing thickness and tends to result in a slight separation, an annulus, between the cement and casing. It is important to know whether the cement is performing its function, i.e. whether the cement bond is hydraulically secure.
Techniques have been proposed to ascertain the quality of the cement bond. In this sense the term "bond" as used herein, is to be understood to include both those cases where the cement actually adheres to the casing as well as when there is no adhesion but instead a small micro-annulus which is so small as to prevent fluid communication between cement separated zones. In other words, the term "good bond" means that separation of zones by the cement is adequate to prevent fluid migration between the zones even in teh presence of a micro-annulus. It is, therefore, desirable that cement evaluation techniques identify such micro-annuli as good cement bonds while recognizing annuli incapable of separating zones as hydraulically insecure or bad bonds.
The problem of investigating the cement behind a thick casing wall with a tool located inside the casing has led to various cement evaluating techniques using acoustic energy.
For example, in the U.S. Pat. No. 3,401,773 to Synnott III a cement logging technique is described using a tool employing a conventional longitudinally space sonic transmitter and sonic reciever. The casing signal traveling through the casing is processed whereby a later portion, which is afftected by the presence or absence of cement, is extracted. The extracted segment is integrated to provide a measurement of its energy as an indication of the presence or absence of cement behing the casing. Although such technique provides useful information about cement defects behind the casing, the evaluation of the quality of the cement bond may not be sufficiently precise since the measurement averages cement conditions over a substantial distance between the transmitter and receiver and does not provide circumferential resolution i.e. information as to the bond condition at various points around the casing. Furthermore, the technique may characterize a hydraulically secure annulus as a defective cement bond because of inadequate energy transfer from the casing signal to the cement through the annulus.
A more precise technique for evaluating the cement condition is described in the U.S. Pat. No. 3,697,937 to Ingram and assigned to the same assignee as for this patent application. Ingram discloses a sonic transmitter-receiver with zero spacing to measure reflection coefficients from reflections produced by material discontinuities. Cement conditions in cased boreholes are evaluated by comparing the relative amplitude and phase of reflected sonic energy impinging upon paired acoustic transducers at a plurality of frequencies. The sonic investigation is described as particularly useful at sonic frequencies in the range from about 5 KHz to 50 KHz. At such sonic frequencies the reflection coefficients (the ratio of amplitudes of incoming waves to outgoing waves in the mud inside the casing) vary as a function of whether there is a cemented or uncemented annulus, the width of the annulus and hardness of the formation.
In the U.S. Pat. No. 3,732,947 to Moran et al an acoustic pulse technique for cement evaluation logging is described wherein the attenuation of acoustic signals reflected from material discontinuities is measured at radially resonant frequencies effectively without circumferential resolution. The measured attenuation constants are then employed to compute the thickness of the annulus and the cement with the computation dependent upon the type of formation as well as upon measurements conducted at different resonant frequencies. This technique employs low frequnecies where compensation for formation characteristics to be obtained from another well log are required. Furthermore, information on the thickness of the cement annulus is needed to derive an evaluation of the annulus between the cement and casing.
When acoustic cement evaluation techniques are carried out at low frequencies such as described in the patents to Ingram and Moran et al, so-called radial or hoop-mode resonances are observed. One resonance includes the casing-annulus system, a second higher resonance occurs for the cement annulus itself. The technique of employing such resonances to sense absence or presence of cement in the annulus around the casing does not lend itself easily to evaluating the cement bond quality in the presence of small casing-cement annuli.
In the U.S. Pat. No. 3,175,639 to Liben, an acoustic pulse echo technique is described to investigate the formation zone alongside a borehole. An acoustic pulse generator operating at a frequency of the order of about 10 MHz is applied adjacent the wall of a borehole and actuated to generate very short acoustic pulses towards the formation. The elapsed time between the transmitted sonic pulse generation and the reflected pulses are measured as well as the amplitude of the returned pulse. The measurements are then used to derive the acoustic impedance of the formation.
In the Liben patent a processing apparatus is described with which the return pulse occurring after the transmitted pulse is rectified and integrated. The integrated signal is indicated as proportional to the average amplitude of the return pulse. The integrated signal is used to derive the acoustic impedance of the formation alongside the borehole with the use of a measurement of the thickness of the mud cake, a knowledge of the amplitude of the transmitter pulse, the absorption characteristic of the mud and the acoustic impedance of the mud cake.
The acoustic pulse echo technique described in Liben does not lend itself well for evaluating the quality of the cement bond. The proposed frequency of operation by Liben is too high, thereby tending to characterize all micro-annuli as poor cement bonds. Furthermor, the acoustic transducer is mounted close to the borehole wall so that secondary transmission interference problems may occur such as when a returned echo is reflected from the transducer as a second transmission back to the formation.
In the U.S. Pat. No. 3,340,953 to Zemanek, an acoustic through-casing formation borehole logging technique is described with acoustic frequencies determined by the casing thickness. The apparatus functions by transmitting acoustic energy from a transmitter to a pair of remotely spaced receivers. The frequency of the acoustic energy is selected on the basis of a particular relationship depending upon the velocity of the shear wave in the casing, an arbitrary dimensionless number and the casing thickness. The suggested transmitter frequencies are from 300 KHz to 460 KHZ for a casing thickness of 1/4 inch thickness and correspondingly lower frequencies for thicker casings.
The Zemanek system does not operate on a specific isolated casing segment but, because of the transmitter-receiver spacing along the borehole, provides an average evaluation over the spacing involved. Zemanek neither describes an apparatus nor a method for investigating the cement bond by analyzing the relections from radially successive interfaces.
The U.S. Pat. No. 3,883,841 to Norel et al describes a similar acoustic pulse echo technique as in Liben for measuring the acoustic impedance of material alongside a wall in a borehole. The acoustic pulse transducer in Norel is provided with different acoustic coupling layers between the flush mounted transducer and the borehole. The Norel et al device suggests employing a source pulse whose frequency spectrum occurs in the range from about 100 KHz to about 5 MHz. This is a frequency range of generally the same bandwidth as proposed in U.S. Pat. No. 2,825,044 to Peterson who suggested an ultrasonic device for exploration of a borehole wall with acoustic waves at frequencies from 100 KHz to 10 MHz.
The acoustic echoes obtained as proposed by Norel et al are stated as useful for checking the cement bond. Norel teaches that to measure the acoustic impedance of the material in contact with the casing, two consecutive peaks of received impulses are to be extracted and their ratio generated for use in a computation network to derive the acoustic impedance. Since a casing thickness may vary in practice as much as from 10% to 20%, the Norel gating approach to extract successive reflection is difficult to implement. Furthermore, the acoustic impedance coupling layers suggested by Norel introduce attentuation. As a result, the potential error in measuring individual reflections is increased, thus reducing the effectiveness of Norel et al's analysis of the acoustic investigation.
In a simplified approach described with reference to FIG. 15, Norel et al propose to check the cement bond by directly integrating the entire received echo signal and recording the resulting integration as a function of depth. This technique includes the strong casing reflection whose inclusion obscures the more significant later reflections and is likely to include formation echoes in well-bonded hard formations.
A frequency range such as proposed by Norel et al includes at the low end frequencies tending to drive the casing-annulus into hoop-mode resonance with the attendant sensitivities which make cement bond evaluations in the presence of small annuli difficult. At the high end of Norel et al's frequency range, the casing-cement annuli are likely to be consistently interpreted as bad cement bonds even though the cement might be hydraulically secure. Furthermore, the spacing between Norel's transducer feeler and the casing tend to appear as a small annulus, thus obscuring the evaluation of the cement bond.
When an acoustid pulse echo technique for investigating a borehole is employed, it is desirable to obtain an adequate number of cycles in the reflected pulses before a secondary interference as herein described with respect to Liben is obseved. When an acoustic pulse transducer as described in Norel et al is mounted flush to the inner wll of a casing, the first echo return occurs very soon and its reflection from the transducer back to the casing causes secondary reflections which tend to interfere with the initial echo signals of interest.
One can introduce special acoustic coupling layers between the transducer and the casing as proposed by Norel et al. With such layers, however, the echo signals tend to be also reduced in amplitude. Furthermore, the proximity of the transducer to the material interfaces reduces the number of echo signals with useful amplitudes before secondary transmission interference arises. Though use of high frequencies such as from one to five MHz enable sharper or shorter duration transmitter pulses, those same frequencies tend to be incompatible for evaluating small casing-cement annuli. Such high frequency sonic waves also tend to be affected by the casing surface whose roughness may cause destructive interference.
When an acoustic pulse producer such as described in Norel et al is employed in an ultrasonic echo testing device as described in Russian Patent No. 405095 or the U.S. Pat. No. 3,974,476 to Cowles, the increased spacing suggested by the latter between the transducer and the casing enables reception of a greater number of cycles. However, in such case the intermediate layers proposed by Norel et al between the transducer and the casing tend to severely attenuate the echo signals which already arrive with reduced amplitude by virtue of the increased spacing.
The U.S. Pat. No. 3,339,666 to McDonald describes an acoustic pulse echo technique for a cased borehole using an acoustic frequency at which the casing appears transparent. The suggested acoustic pulse frequency range is about 100 KHz, with a particular range suggested between 200 to 400 KHz. The reflections are transmitted from the borehole tool to the surface where all of the reflections occurring after a gating time of about 100 microseconds following the firing and before the next succeeding acoustic pulse from the transmitter are rectified, integrated and recorded.
McDonald characterizes the reflection segment selected for integration and recording as representative of the acoustic impedance of the formation. In practice, however, significant reflections from the formation at the casing thickness resonance frequency occur in limited situations such as when the cement is well bonded to both the casing and the formation and when the formation itself can provide a strong reflection. Formation reflections tend to be cluttered by secondary transmission effects, such as when an initial acoustic reflection from the inner wall of the casing causes a secondary transmission when partially reflected off the face of the transducer.
When the borehole wall is rough or has craters or crevasses, as frequently occurs, the formation acoustic reflections tend to be scattered and quite weak by the time they arrive at the acoustic transducer. When the cement annulus is not properly bonded to the casing and formation, further attenuation and scattering of the formation reflection is likely, resulting in further weakening or complete loss of the formation reflection.
McDonald further proposes the transmission of the reflection through suitable conductors in a cable. Techniques for the transmission of high frequency signals of the order of 500 KHz such as occur in the reflection signal are well known. Well logging cables, however, are typically limited to signals whose frquencies occur below about 100 KHz. As a result, a high frequency reflection signal attributable to reverberations between the inner and outer casing walls would be highly attenuated by the cable.
It is important in well logging operations to obtain information as to the current condition of the casing employed in boreholes. The installed casing may be exposed to various corrosions due to chemically active corrosive solutions, electrolytic corrosion due to ground currents or contact between dissimilar metals. Corrosion of the outside casing wall may result in a highly undesirable hydraulic communication between formation zones which must remain isolated from each other by the cement. Excessive wear may arise due to abrasion from fluid flows. Hence, over a period of time, the borehole casing may deteriorate with excessively thin and weakened resions. Such deterioration can be harmful causing collapse of the protective casing and perhaps loss of the well or, if leaks develop in the casing, uncontrolled movement of fluids within the well and adjacent formations. Unlike well tubing, once casing is installed in a well, it is difficult or impossible to remove the casing for inspection. It is, therefore, particularly useful to be able to inspect the casing in situ to determine the presence and location of bad casing conditions.
Ultrasonic pulse echo techniques for determining the thickness of materials have been extensively proposed in the art. Commencing, for example, with the U.S. Pat. No. 2,538,114 to W. P. Mason, an apparatus is described for measuring the thickness of a material by noting its resonance frequency when the material is irradiated with ultrasonic energy. In the U.S. Pat. No. 2,848,891 to J. E. Hunter et al, a technique is described whereby the grain size of materials is measured by observing the ultrasonic frequency response of the material. In the U.S. Pat. No. 3,595,069 to Fowler et al a system is disclosed whereby an ultrasonic sensor is stimulated into a resonance and the resonance frequency measured to detemine the value of the parameter for which the sensor is used. In the U.S. Pat No. 4,003,244 to O'Brien et al, the thickness of a material is measured by employing a pulse echo technique.
Various frequency domain techniques have been employed in acoustic investigations to determine the thickness of materials. For example, in an article entitled "Ultrasonic Signal Processing Concepts for Measuring the Thickness of Thin Layers", published at page 249 of the December, 1974 issue of Materials Evaluation by J. L. Rose and P. A. Meyer, a frequency analysis is described for determining the thickness of a thin layer. As described in this article, an input acoustic pulse is applied with sufficient bandwidth to cover the fundamental or harmonic resonance frequency of a thin layer placed between two materials. A spectral profile of the echoes from various layers is made as illustrated in FIGS. 11 and 12 of this article. With particular reference to the broadband frequency spectra shown in FIG. 12, dips in the frequency spectrum occur at those frequencies which bear a particular relationship to the thickness of the material being measured. The center frequency of such dips, however, are not conveniently measured, particularly when the frequency spectrum of an echo reveals several dips.
Acoustic techniques have been described with which a plate, whose thickness is to be measured, is driven into a thickness resonance by utilizing a feedback of resonating vibrations. One such technique is described in U.S. Pat. No. 3,741,334 which issued to W. Kaule.
Kaule describes a particular ultrasonic technique for determining the thickness of a plate by measuring its thickness resonance. Resonance is induced in the plate by first subjecting the plate to a noise source for a first interval and recording the decaying free resonance ultrasonic sound during a second subsequent interval. After the plate has ceased resonating, the previously stored sound is played back and used to induce resonant vibrations in the plate followed by a subsequent recording of the decaying sound after the second inducement. This process is repeated to achieve a high amplitude resonance in the plate and enable a measurement of the plate's resonance frequency and thus the plate's thickness. Frequency is measured by counting the amplitude peaks of the decaying resonating vibrations over a particular interval or by determining the time needed to count a particular number of peaks.
An alleged improvement over the Kaule U.S. Pat. No. 3,741,334 is described in U.S. Pat. No. 3,914,987 to Bickel et al. The improvement appears to relate to use of a bidirectional counter and a delay, but determination of the resonance frequency still involves the counting of individual peaks in the decaying vibrations from the resonating plate.
When an acoustic pulse echo technique is used to determine the thickness of a casing cemented in a borehole penetrating an earth formation, the acoustic returns have a complex form. A waveform representative of such acoustic return is illustrated in FIG. 4 herein and shows that a reliable peak to peak frequency determination is at best difficult and more likely impractical. Furthermore, the casing bore is circular tending to produce acoustic interferences from reflections of surface irregularities and the like; thus further cluttering acoustic returns.
In addition, the time available for the investigation of the thickness of any one small casing segment is limited if an extensive investigation of the casing is to be completed within a resonable time. Hence, the time needed to execute an acoustic feedback investigation of the type described in the Kaul and Bickel et al patents does not in practice appear tolerable.
In an article entitled "Broad-Band Transducers, Radiation Field and Selected Applications" by E. P. Papadakis and K. A. Fowler and published at page 729 of Vol. 50 Number 3 (Part 1) of the 1971 issue of The Journal of the Acoustical Society of America, a pulse induced resonance technique is described for determining the thickness of a thin material. The technique describes a selective time-domain gating of pulses reflected by the thin material and an analysis of their frequency content with a spectrum analyzer. An automatic tehcnique for deriving the thickness of the thin material is not described.